Would a hard-coded (not in user-space) page manager (inside Site Manager) be sufficient? It could allow easy bulk tagging, renaming, deleting etc.

Yes, this would be sufficient. The current userspace navigators serve as a kind of prototype. We'd need a compact list of pages sortable by various criteria, and additionally, a way to select individual or all pages, then act on that selection:

It would be useful to be able to select pages in a page-tree view and to be able to select whole or partial sub-trees. Along with tag buttons, which is now accepted, this would then make practical a 100% Wiki-based forum.